The present invention relates to an error correcting apparatus in position detection, which is installed in a position detector for detecting a position on a linear position or a rotational position.
FIG. 1 is a block diagram showing a conventional position detector and error correcting apparatus in position detection. FIG. 2 is a graph showing position detection errors in the position detector shown in FIG. 1.
In FIG. 1, there is provided an one-revolution absolute position detector comprising an input shaft 1, a 4.times.-resolver 2, a 1.times.-resolver 3 and a digitizing circuit 4. This position detector enables the 4.times.-resolver 2 to perform an absolute detection with a high resolution up to one-fourth revolution around the input shaft 1. Further, this position detector allows the 1.times.-resolver 3 to revise the position detection data detected by the 4.times.-resolver 2 to data which indicate an absolute position within one revolution around the input shaft 1. With this arrangement, the position detector shown in FIG. 1 detects a rotational position in one revolution of the input shaft 1 with a resolution up to 1/65536 revolution.
On the other hand, a PROM (programmable ROM) 5 holds, or is written in with, data obtained from the measurement which has in advance performed every 1/256 revolution as to the position detection errors (the graph shown in FIG. 2) within one revolution in the position detector as shown in FIG. 3. The detected rotational position data are outputted as numeral data .theta..sub.e through the digitizing circuit 4 onto a microcomputer 6, which, in turn, based on the detected value .theta..sub.e from the digitizing circuit 4, reads out from the PROM 5 the data on the position detection errors before and after the detected value. Then, the microcomputer 6 subjects the read-out data to interpolation operation as shown in a formula (1) to determine the measurement error e of the detected value .theta..sub.e, and using a formula (2), calculates and outputs rotational position data .theta. free from the error. EQU e=E[.theta..sub.e /256]+(E[.theta..sub.e 256+1]-E[.theta..sub.e /256]).times.(.theta..sub.e MOD 256)/256 (1) EQU .theta.=.theta..sub.e -e (2)
In the conventional error correcting apparatus in position detection described above, in the case where a precision measurement is required using a position detector with high resolution, the less the error in the interpolation operation becomes, the more the error in the position detection increases. As a result, to deal with such a case, it is necessary to provide a non-volatile memory having a large capacity in order to store the data on position detecting errors. This requires, however, a PROM which is unable to be revised after writing and a high cost large capacity EEPROM (electrically erasable PROM), and further gives rise to a difficulty in that the devices are too large to be incorporated. Moreover, the measured data on position detecting error are liable to be mixed with noises during the measurement, an therefore performing a correction based on the data disadvantageously causes errors in accordance with the mixed noises to the corrected position detection values.